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Last updated: September 23, 2025

The Toxic Legacy of Acid Rain

Forest damaged by acid rain

Acid Rain: A Silent Threat

Acid rain is one of the most persistent environmental legacies of the industrial era. This phenomenon, first identified in the 19th century by Scottish chemist Robert Angus Smith (1817-1884), reached dramatic proportions in the 20th century, becoming a major global environmental issue.

The 19th-century Industrial Revolution marked the beginning of a significant increase in acidifying pollutant emissions. The work of Swedish scientist Svante Odén (1924-1986) in the 1960s established the link between industrial emissions and the acidification of Scandinavian lakes, alerting the international scientific community.

Acidity peaked in the 1970s and 1980s in Europe and North America, with pH values dropping as low as 4.0, or even 3.0 in heavily industrialized regions.

The Mechanism of Acid Rain: pH and Precipitation

Acid rain refers to any form of precipitation with a pH below 5.6, the natural acidity level of rainwater in equilibrium with atmospheric carbon dioxide.

Its formation is primarily due to atmospheric pollutants such as sulfur dioxide (SO2) and nitrogen oxides (NOx), mainly from the combustion of fossil fuels.

These compounds undergo oxidation reactions in the atmosphere, transforming into strong acids: pH < 1, as strong acids like sulfuric acid (H₂SO₄) and nitric acid (HNO₃) typically have very low pH levels, often below 1 when concentrated.

\( \text{SO}_3 + \text{H}_2\text{O} \rightarrow \text{H}_2\text{SO}_4 \) (sulfuric acid)
Sulfur trioxide molecules attach to water droplets in the atmosphere. This reaction represents the hydration of SO₃ to form sulfuric acid. SO₃ is an acidic oxide that reacts violently with water. This is the primary source of acidity in acid rain.

\( 2\text{NO}_2 + \text{H}_2\text{O} \rightarrow \text{HNO}_3 + \text{HNO}_2 \) (nitric and nitrous acids)
Nitrogen dioxide gas dissolves in water. This reaction is a dismutation where nitrogen dioxide (NO₂) acts as both an oxidant and a reductant. Nitric acid (HNO₃) is stable and contributes durably to acidity. Nitrous acid (HNO₂) decomposes in sunlight and can recreate highly reactive particles that clean pollutants from the atmosphere.

pH Scale of Precipitation
Type of PrecipitationpH RangeCharacteristicsObservations
Pure Water (reference)7.0NeutralTheoretical reference value
Unpolluted Rain5.6 - 5.0Slightly AcidicNatural acidity due to atmospheric CO2
Mild Acid Rain4.9 - 4.3AcidicFirst signs of acid pollution
Moderate Acid Rain4.2 - 3.5Very AcidicMeasurable ecological impacts
Severe Acid Rain3.4 - 2.5Extremely AcidicSignificant environmental damage
Historical Record2.4ExceptionalPitlochry, Scotland (1974)

Source: U.S. Environmental Protection Agency and European Environment Agency.

Ecological Impacts of Acid Rain

Acidic smoke

Impacts on Aquatic Ecosystems

The acidification of lakes and rivers causes the release of toxic aluminum (Al³⁺) from soils, leading to the gradual disappearance of many fish and aquatic invertebrate species.

Impacts on Forests and Soils

Acid rain leaches essential nutrients such as calcium (Ca) and magnesium (Mg2+) from soils while mobilizing toxic heavy metals like aluminum. This phenomenon caused forest decline (Waldsterben) in Central European forests during the 1980s.

Impacts on Monuments and Buildings

Sulfuric acid (H2SO4) reacts with limestone (CaCO3) in monuments: \( \text{CaCO}_3 + \text{H}_2\text{SO}_4 \rightarrow \text{CaSO}_4 + \text{CO}_2 + \text{H}_2\text{O} \)
This reaction accelerates the erosion of historical architectural heritage.

SO2 Emissions Status in 2025

Faced with the scale of the problem, several international agreements were concluded, including the Convention on Long-Range Transboundary Air Pollution (1979) and its Gothenburg Protocol (1999), which imposed drastic reductions in SO2 and NOx emissions.

Flue gas desulfurization and selective catalytic reduction techniques have led to a significant decrease in emissions in industrialized countries, with a reduction of over 70% in SO2 emissions in Europe since 1990.

Evolution of SO₂ Emissions in Europe (1990-2020)
YearSO2 Emissions (million tons)Reduction Compared to 1990Main Measures
199025.30%Baseline level
200012.849.4%First desulfurization installations
20107.470.8%Widespread use of filters and strict standards
20204.283.4%Energy transition and coal plant closures

Source: European Environment Agency and United Nations Economic Commission for Europe.

Current Perspectives and Future Challenges

Developed countries have largely controlled the acid rain problem

A Contrasting Global Situation

While developed countries have largely controlled acid rain, the situation remains concerning in rapidly industrializing regions, particularly in Asia. China, for example, experienced an acidification peak comparable to Europe's in the 1980s before implementing drastic reduction policies starting in the 2010s.

The Persistence of Long-Term Effects

The current challenge lies in the persistence of long-term effects, with slow recovery of aquatic and forest ecosystems, sometimes requiring active neutralization operations.

Liming: An Essential Corrective Solution

Liming soils and lakes is an essential corrective solution to mitigate the effects of acid rain. This technique, widely used in Scandinavia and Central Europe, involves spreading limestone to neutralize persistent acidity. Although liming is only a palliative response to pollutant emissions, it has saved countless aquatic and forest ecosystems while waiting for source reductions in acidifying emissions.

Current Global SO2 Emissions

Global SO₂ Emissions (1990 vs 2023)
Region/Country1990 Emissions (Mt/year)2023 Emissions (Mt/year)ChangeTechnology Status
European Union25.31.8-93%Advanced technologies widely adopted
United States15.71.9-88%Strict standards, coal plant closures
China12.58.2-34%Massive deployment since 2010
India3.89.1+140%Limited deployment, rapid industrial growth
Russia9.23.5-62%Partially modernized technologies
South Africa1.61.9+19%Old equipment, coal dependence
Latin America4.13.2-22%Uneven progress across countries

Conclusion

Acid rain represents a textbook case of the environmental impacts of industrialization, but also demonstrates humanity's ability to address major ecological challenges through international cooperation, technological innovation, and regulation. This poisoned legacy reminds us of the importance of a preventive approach to new environmental challenges, particularly climate change.

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